Inflammatory syndromes such as sepsis and acute lung injury evoke complicated immune responses from the host. When such events occur in succession, the outcome may be even more severe than when they occur alone. This may be because the first hit primes the immune system for an exaggerated response to the second insult. However, the responses to two-hit injury have not been uniform in either clinical or experimental studies. One explanation for this may be that the immune status of the host varies with the interval between the hits and thus affects the subsequent response. In fact, our preliminary data suggests that neutrophil recruitment to the lung when sepsis is followed by acid aspiration varies significantly depending upon the interval between the two insults. It is the goal of this proposal to determine the mechanisms driving neutrophil recruitment to the lung when acid aspiration occurs at various points in the course of septic peritonitis. The hypothesis is that sepsis modulates the pulmonary injury from acid aspiration by control of neutrophil recruitment and/or function through altered chemokine production and CXCR2 expression. In this study, the first specific aim will determine if differences in neutrophil recruitment in response to a direct acid lung injury delivered at different times after cecal ligation and puncture are due to altered gradients of chemotactic factors and/or CXC chemokine receptors. The role of the receptor will be confirmed confirmed by blocking and knockout experiments. The second specific aim will examine the effect of sequential insults on the functional characteristics of the neutrophils including chemotaxis, adhesion molecules, respiratory burst, and proteases. Finally, the third specific aim will examine chemokine concentrations and receptor expression in hospitalized patients suffering from multiple insults. The results of this study will help determine when and where to direct therapy for lung injury that follows sepsis. Coupled with the other proposals in this grant, this study will improve the understanding of the immune responses in sepsis with the goal to improve patient management and outcome in the intensive care unit.